1. Field of the Invention
The present invention relates to an apparatus and method for estimating a Carrier-to-Interference-and-Noise Ratio (CINR) serving as a channel quality criteria in a communication system based on an Orthogonal Frequency Division Multiplexing (OFDM) or an Orthogonal Frequency Division Multiplexing Access (OFDMA).
2. Description of the Related Art
The Orthogonal Frequency Division Multiplexing (OFDM) technique has recently been used to transfer data at a high rate through a wired/wireless channel. The OFDM technique transfers data by means of a plurality of sub-carriers. The OFDM technique converts input serial data into parallel data, modulates the parallel data into a plurality of sub-carriers, that is, sub-channels, with orthogonality, and transfers the modulated data.
This OFDM technique is widely applied to digital transfer technologies such as digital/audio broadcasting, digital TV, Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode (WATM), Broadband Wireless Access (BWA), etc. In the past, the OFDM technique was not widely used due to hardware complexity, but has been recently employed after the development of various digital signal processing technologies including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT). This OFDM technique is similar to a conventional Frequency Division Multiplexing (FDM) technique, but above all can achieve optimal transfer efficiency by transferring a plurality of sub-carriers while maintaining the orthogonality therebetween. Moreover, the OFDM technique can improve the frequency efficiency and is robust against the effects of multipath fading. Further, the OFDM technique is robust against the effects of frequency-selective fading by making use of overlapping frequency spectra and can reduce the effect of intersymbol interference by making use of guard intervals. In terms of hardware, the OFDM technique can employ a simple equalizer and is robust against the effects of impulse noise.
In a communication system based on OFDM/OFDMA, channel signal quality parameters used for Adaptive Power Control (APC), adaptive modulation/demodulation, etc., for example, a Carrier-to-Interference-Noise Ratio (CINR), must be measured. An APC or adaptive modulation/demodulation unit controls power or a modulation/demodulation level according to the measured channel signal quality using a CINR value. The CINR is defined by the total sum of sub-carrier signal power divided by the total sum of noise and interference power, and serves as a criterion for determining channel quality in the communication system.
The prior art for estimating the CINR is disclosed in U.S. Pat. No. 6,456,653 (hereinafter, referred to as the “'653 patent”) entitled “FAST AND ACCURATE SIGNAL-TO-NOISE RATIO ESTIMATION TECHNIQUE FOR OFDM SYSTEMS”. The '653 patent provides a method for estimating a noise level from unused sub-carriers. An OFDM system performs an IFFT operation on data to be sent by a transmitter and transmits a result of the IFFT operation. When an IFFT size corresponds to N points, only Aused sub-carriers are used to encode the signal for transmission to a receiver, and the remaining (N−A)unused sub-carriers are nulled. The Aused sub-carriers from among the signals output after an FFT operation by a receiver include data mixed with noise, and the remaining (N−A)unused sub-carriers include only noise. In the '653 patent, the noise level is measured from the (N−A)unused sub-carriers. Under the assumption that the measured noise level is identical to a level of the noise mixed with the data, a true signal level is measured when the measured noise level is subtracted from a level of power received from the Aused sub-carriers. A ratio of the true signal level to the noise level becomes an estimate of the targeted Signal-to-Noise ratio (SNR). This SNR is used to determine channel quality in the communication system such as the CINR.